Field of the Invention
The present invention relates to a digital to analog converter (DAC), and more particularly to a digital to analog converter with output impedance compensation for enhancing performance of digital to analog conversion.
Description of the Prior Art
With reference to FIG. 13, a multi-bit current-steering digital-to-analog converter (DAC) is composed of multiple current cells 200. Each of the current cells 200 includes a current source Iu with an output impedance Zo of the current cell 200 in parallel and a pair of current switches, wherein the pair of current switches is simplified as a single switching element 202 with two output ends in FIG. 13. Ideally, the output impedance Zo of the current cell 200 should be infinitely large, so the current from the current source Iu can fully flow to an output load RL instead of flowing to the output impedance Zo. However, the output impedance of any practical device is finite. As a result, the output current of the current source Iu does not fully flow to the output load RL, causing some non-ideal effects.
For a single current cell 200, the finite output impedance Zo will only cause gain error rather than nonlinear distortion if the value of the output impedance Zo is constant. However, the number of all the current cells 200 in the multi-bit current-steering DAC is much more than one. For an N-bit current-steering DAC, there are 2N−1 current cells 200. All of the current cells 200 are connected in parallel to sum their output currents at two output terminals Vout+, Vout− of the DAC.
With reference to FIG. 14, considering an N-bit current-steering DAC, there are 2N−1 current cells 200 connected in parallel at the output terminals of the DAC. An overall output impedance Zout seen from the output terminals Vout+, Vout− of the DAC is correlated to the number of the current cells 200 connected at the output terminals Vout+, Vout−. The connection of each current cell 200 to any one of the output terminals Vout+, Vout− is determined by its respective current switch, which is controlled by digital input signals B1, B2 . . . BN. As a result, the overall output impedance Zout is nonlinear and varies with the digital input signals B1, B2 . . . BN. The nonlinear overall output impedance Zout will cause harmonic distortion tones in frequency domain.
In addition to the input signals-dependent nonlinearity, the output impedance Zo of each current cell 200 is also affected by an output voltage at the output terminals Vout+, Vout−. The output voltage dependency of the output impedance further degrades the linearity of the DAC.